With the discovery of fields producing hydrocarbons located in the sea at ever-greater depths the utilization of rigid structures fixed to the seabed to support production installations has become more and more costly.
Consequently, the use of floating structures (better known among specialists as Stationary Production Units—SPUs to receive production installations has been emerging as an ever more-frequent alternative that normally presents a lower cost than fixed structures.
These Stationary Production Units (SPUs) are connected to ascending fluid production and export tubes also known among specialists by their English language designation as “risers.” Risers conduct the production of producing wells to the SPU, or link the SPU to other installations that receive the production of the fluids produced.
These Stationary Production Units (SPUs) must have such characteristics as to allow the utilization of ascending fluid production and export tubes (“risers”) in catenary curves made of steel, also known among specialists by the English language abbreviation “SCR,” derived from “steel catenary risers.”
These steel catenary riser tubes (SCRs) present a lower manufacturing and installation cost in comparison to the flexible riser tubes used to that point. Furthermore, in the case of production at greater depths, they lighter, which reduces the load to be supported by the floating structure.
Frequently the economic feasibility of an enterprise for the exploitation of hydrocarbon-producing maritime field depends upon the reduction of total installation and operation costs. Specialists have proposed new Stationary Production Unit (SPU) shells to meet this requirement.
Significant technological innovations have emerged in recent years in anchoring systems and in ascending fluid production and export tube (“riser”) systems. Such innovations, however, have been applied to conventional structures, such as semi-submersible platforms and tanker ships in the majority of cases, or to a few structures of a mono-columnar type of enormous draft, known to the specialists as a SPAR type platform, or else to platforms with far-reaching legs, known as TLP type platforms.
Semi-submersible platforms have the principal characteristic, in addition to being anchored, of remaining in a substantially stable position, presenting small movements when they suffer the action environmental forces such as the wind, waves and currents.
This type of floating structure has some disadvantages to being utilized as an SPU. An outstanding disadvantage is its limited load-bearing capacity, which often limits the use of equipment in processing plants installed on the platform deck. Another is a low storage capacity for storing the oil that comes on board, also arising from the limited load-bearing capacity.
Another problem to be noted is the fact that the hull of semi-submersible platforms are structures that are difficult to manufacture, and require techniques different from those employed in ship construction. This type of floating structure also has difficulty in absorbing variations in the load on deck that arise from project changes, which results in a great disadvantage in its use.
Project alterations arise from necessities that emerge during the development of a project, and from employing more equipment in the processing plant as the result of re-evaluation of the production capacity of the field producing the petroleum where the floating structure will be employed.
The use of tanker ships adapted to receive a production plant on their deck has become quite common in recent years, specifically due to the great availability of this type of floating structure on the market. The great advantages presented by such ships are their great capacity to receive loads and their great capacity for storage.
Tanker ships, however, have the disadvantage of not being axi-symmetrical structures and the load on the ship's structure is going to vary in function of the locations where the various items of production equipment are mounted.
The great disadvantage in using a ship to house a production plant on its deck is related to the difficulties that this type of floating structure presents to operating in a stabilized manner in relation to environmental conditions, i.e., winds, waves and marine currents.
For ships to be able to receive ascending fluid production and export tubes in a stable manner, i.e., without significant movements in such tubes, it is very common to employ a rotational structure in the ship that is provided with a body anchored to the seabed. That body remains in a substantially fixed position in relation to the seabed, presenting quite limited movements.
The aforesaid body is provided with roller bearings that are fixed to the ship, about which the ship may rotate on the rotational axis of the fixed body. The fluid production and export tubes are connected to the aforesaid body and the body is provided with rotational connections that allow the fluids to flow through to the production plant installed on the deck of the ship.
This type of rotating structure is known among marine oil field technicians by its English language nomenclature: “turret.” The turret is installed in an existing internal opening in the hull of the ship, also known among the technicians by its English language nomenclature: “moonpool.”
This type of arrangement presents the disadvantages of being very expensive and requiring great care in its operation. Furthermore, the dynamic impediments in the rotating connections are a constant concern, since a possible gas leak at that point would endanger the entire installation.
Also known are some structural concepts derived from SPAR and TLP type platforms, about which very little is known of any actual use since, in order to be utilized as alternatives for production in deep waters they still require the implementation of exhaustive studies and the execution of the corresponding engineering project.
The SPAR type platform, which refers to a projected floating type structure projected to undergo few movements, presents the inconvenience of being provided with an extremely deep draft, about 150 meters, along with the fact that varying its draft is impossible.
The great disadvantage of the SPAR type platform is related to the fact that its extremely deep draft makes it impossible to transport the platform with the deck for installation of the equipment in place. It is therefore necessary to carry out the mounting of the deck in the open sea, which requires the use of various maritime support vessels, all quite sophisticated and of an elevated operational cost.
Document WO 02/090177, published Nov. 14, 2002, appended hereto for reference, describes a type of floating structure that includes an axi-symmetrical platform cylindrical or polygonal in shape, with a flat bottom. This floating structure is provided with peripheral indentation in its lower, submerged part, and the possibility of storing oil inside the structure is projected.
The basic function of the lower peripheral indentation, as described in Document WO 02/090177, is to endow the floating structure with a capacity to attenuate the effects that waves exercise on it. The peripheral indentation may be provided with deflecting and perforated plates to increase the attenuation effect against the force of the waves.
The floating structure described in Document WO 02/090177 may also be provided with an internal opening (“moonpool”) to receive ascending fluid production and export tubes (“risers”) or with the drilling column of a deep-sea drilling line or probe, with all its complementary equipment.
The great advantage provided by this type of structure lies in its tremendous capacity to receive a load, associated with the fact that it is a floating structure with minimal movements, something that facilitates production and drilling operations.
Although the description of the floating structure set forth in Document WO 02/090177 does not allow us to conclude with certainty, it appears that the floating structure is quite large, with a tremendous displacement, in order to operate in an adequate manner.
The present invention relates to a floating structure for the reception of production and/or drilling installations on its deck so as to resolve all the problems related above.